In a single-sheet printer or a copier to which a respective, single sheet of the recording medium, for example a single sheet of paper, is supplied for printing or, respectively, copying, there is often the problem that the recording medium is drawn in into the single-sheet printer or, respectively, copier at an angle or offset. Due to the angled or offset draw-in of the recording medium, this assumes a position that deviates from a rated printing position wherein the recording medium can be properly printed. Since a proper printing of the recording medium is only possible in the rated printing position, it is particularly pre-print forms or recording media that are already printed with ink and onto which successor colors are to be printed can no longer be printed error-free.
Various devices are known for solving this problem. U.S. Pat. No. 4,805,895, thus, discloses a means in a printer with which a sheet that is displaced transversely relative to the rated printing position can be aligned. The means is formed of a delivery device and of a transport device that follows this as viewed in conveying direction and that can be moved transversely relative to the conveying direction. During the transport of the sheet, the delivery device is operated at a higher conveying speed than the transport device, so that a loop forms between the two devices, this loop enabling a movement of the transport device transversely relative to the conveying direction without damaging the sheet to be aligned. For aligning the sheet, the transport device transporting the sheet to be aligned is moved transversely relative to the conveying direction in conformity with the previously identified offset.
EP-A-0 658 503 discloses an aligning device for copier devices with which a sheet drawn in at an angle with respect to the conveying direction can be aligned parallel to the conveying direction. The aligning device is formed of two driven roller pairs that are arranged on a common line transversely relative to the conveying direction and respectively seize an edge region of the sheet to be aligned. For aligning the sheet parallel to the conveying direction, the two roller pairs are driven with different conveying speeds, so that the one edge region of the sheet is retarded or accelerated compared to the other edge region of the sheet.
DE 32 23 048 C2 discloses an aligning device for copier devices wherein the position of individually supplied sheets of a recording medium can be modified before printing with a conveyor means displaceable transversely to the conveying direction. Given this known aligning device, the sheet is first aligned parallel to the conveying direction. To that end, the conveying nip of the conveying device proceeding transversely, i.e. roughly perpendicularly to the conveying direction is closed, so that a contact line forms between the conveyor units. This contact line between the conveyor unit forms the stop for the leading edge of the sheet. When a recording medium sheet is then placed into the copier device, it is drawn in with the delivery device preceding the conveyor device and is conveyed in the direction of the conveyor device. The delivery device only ends the conveying event when the sheet arcs between te delivery device and the conveyor device. As a result thereof, the arced recording medium sheet is under tension and has its leading edge lying uniformly against the contact line between the conveyor units of the conveyor device. Subsequently, the sheet is ceased by the conveyor device and conveyed into the copier device. During this conveying event, a sensor device acquires the lateral edge of the sheet and identified the position thereof transversely relative to the conveying direction and relative to the rated printing position. When the identified position transversely relative to the conveying direction does not correspond to the rated printing position, the conveyor device holding the sheet is stopped and is displaced transversely relative to the conveying direction to such an extent that the position of the sheet corresponds to the rated printing position. The conveying event is subsequently continued.
Given this known aligning device, the distance between the delivery device and the conveyor device following the transport device that transports the sheet into the copier device must be greater than the maximally possible length of a single sheet. Only in this way is it assured that neither the delivery device nor the conveyor device hold the sheet while it is being aligned transversely relative to the conveying direction by the transport device. Otherwise, the sheet would be damaged during the alignment. Consequently, the structural length of the aligning device is dependent on the maximum sheet length and is fashioned correspondingly large. At the same time, shorter sheets that are less than half as long as the sheets with the maximum length are not transported by the aligning device, since the handover of such a short sheet from the delivery device to the transport device or from the latter to the conveying device fails.
Therefore, there is a need for an aligning device whose structural length is comparatively slight and that can align sheets of greatly differing length in a simple way.